Generally, eco-friendly vehicle technology is developing, and vehicle manufacturers are accordingly developing eco-friendly vehicles to achieve environmental and fuel efficiency. Accordingly, the vehicle manufacturers have developed electric vehicles (EV), hybrid electric vehicles (HEV), fuel cell electric vehicles (FCEV) and as future vehicle technologies.
Since these types of environmentally friendly vehicles have a number of technical constraints such as weight and cost, etc., vehicle manufacturers have focused on a hybrid vehicle as a solution to the practical concerns such as emission regulation and the need to improve fuel efficiency. The hybrid vehicle has two or more power sources that may be combined in different ways. The power sources may be conventional gasoline engine or diesel engine using fossil fuel, combined with electrically-driven motor/generator.
The hybrid vehicle uses motor/generator with improved low-torque characteristic at lower speed as a main power source, and uses engine with improved high-speed torque characteristic at higher speed as a main power source. Accordingly, hybrid vehicle has an excellent effect of enhancing the fuel efficiency and reducing exhaust emissions since the engine using fossil fuel is stopped and motor/generator is used in the lower speed range.
A power transmission device for the hybrid vehicle described above is classified into a single-mode type and a multi-mode type. The single-mode type has an advantage of not requiring a torque transmission device such as a clutch and a brake for speed change control, but it also has the drawbacks such as low fuel efficiency due to deteriorating efficiency at high speed driving and need for additional torque pump-up device for applications in large vehicles. The multi-mode type has advantages such as high efficiency at high speed driving, and applicability for various vehicle sizes since it can be designed to pump up torque autonomously.
Therefore, in recent years, the multi-mode type has been mainly adopted, rather than single-mode type. The multi-mode type power transmission device includes a plurality of planetary gear sets, a plurality of motor/generators used for imparting motion and generating electricity, a plurality of torque transmission devices (friction elements) to control the rotation elements of the planetary gear sets, and battery used as an energy source for the motor/generator.
The multi-mode type power transmission device has different working mechanisms based on the connection configuration of the planetary gear sets, the motor/generator, and the torque transmission device. The conventional hybrid vehicle implements motor torque control methods to prevent the shifting shock or to reduce shifting shock generated during mode change.
However, the motor torque control has the following drawbacks. First, torque control using a motor (MG1) may have increased current consumption to apply torque at 0 rpm, which in turn may cause problems such as overheating and deteriorating durability of the battery, inverter or motor. Second, releasing the OD brake during regenerative braking may cause the energy generated from MG2 to be discharged to MG1, which may degrade system efficiency. Lastly, the torque control of the MG1 (motor) may have degraded drivability and riding comfort since it is not possible to perform control when the SOC is depleted.